Microtubules are an integral part of the cytoskeleton of all eukaryotic cells. These structures participate in a vast number of cellular functions, including mitosis morphogenesis, etc. The central core of the microtubule is composed of tubulin, a highly conserved heterodimeric protein. Microtubule assembly can be disrupted by a variety exogenous substances. The most well known is colchicine, an alkaloid from the Autumn crocus. Colchicine inhibits microtubule assembly and subsequently microtubule-mediated process by binding to a single high affinity site on the tubulin heterodimer Another class of microtubule interrupters includes taxol, an antitumor agent from t plant Taxus brevifolia, which interacts with tubulin in a manner different from colchicine. Rather than inhibiting the formation of microtubules, taxol enhances tubulin polymerization into microtubules. One of the important endogenous substances that bind to tubulin is GTP (two sites per dimer, one of which is exchangeable). There are so indications that these ligand binding sites are interrelated. For example, ligand binding the colchicine site generally activates a weak GTPase activity in tubulin, and induces GTPase activity in the tubulin-colchicine complex. In spite of the large amount of the information linking the ligand binding sites to each other, there is no clear picture of precisely how these sites are interrelated. One goal of our research is to determine the spatial relationships between important ligand binding sites on tubulin. we plan to build fluorescent analogs of colchicine, taxol and GTP to tubulin determine the distances between the ligands using fluorescence resonance energy transfer (FRET) .